As a result of rapid increase in recording density in an HDD, the conventionally employed in-plane magnetic recording system is facing severe difficulties in overcoming the problem of “thermal fluctuation”. The thermal fluctuation is a phenomenon in which the recorded signals cannot be stably held, and in the in-plane magnetic recording system, tends to increase in association with the increase in recording density. A perpendicular magnetic recording system on the other hand has, in contrast to the in-plane magnetic recording system, a characteristic where the stability of bits enhances with the increase of the recording density. Thus, development of a perpendicular magnetic recording system is being actively pursued.
A perpendicular magnetic recording medium is mainly composed of a magnetic recording layer of a hard magnetic material, an underlayer for aligning the magnetic recording layer in an aimed orientation, a protective layer for protecting the surface of the magnetic recording layer, and a soft magnetic layers including a soft magnetic backing layer that serves a function to concentrate the magnetic flux generated by a magnetic head for recording in the recording layer. Because direct contact between the magnetic recording layer and the soft magnetic backing layer is known to generate noise due to the interaction between the two layers, a nonmagnetic layer is preferably disposed between the magnetic recording layer and the soft magnetic backing layer.
One of the basic characteristics of a magnetic recording medium is a signal-to-noise ratio (SNR). To improve the SNR, the output from the magnetic recording medium needs to be increased and the noise needs to be decreased. One of the reasons for the decrease in output and the increase in noise is degradation in the variance of alignment (variation of crystal orientation) in the magnetic recording layer. In a perpendicular magnetic recording medium, the axis of easy magnetization of the magnetic recording layer needs to be aligned perpendicularly to the medium surface. If the variance of alignment of the axis of easy magnetization increases, the magnetic flux in the perpendicular direction decreases and the output signal lowers. The present inventors' studies further revealed that a medium with a large variance of alignment deteriorates magnetic isolation between the magnetic crystal grains composing the magnetic recording layer and the swelling of the magnetic cluster size, increasing media noise. See Development and problems in CoPtCr—SiO2 perpendicular media (Japanese) by Shunji Takenoiri, et al., Preprints for 135th Study Meeting, Magnetic Society of Japan, March 2004, pp. 9-16, for example. Therefore, the variance of alignment in the magnetic recording layer must be as small as possible to enhance output and reduce noise in a perpendicular magnetic recording medium.
In addition to the above-described requirements, the size of magnetic crystal grains in the magnetic recording layer must be reduced for noise reduction in a perpendicular magnetic recording medium. Large magnetic crystal grains make the transition region of the recording bits zigzag shaped, increasing the transition noise. Consequently, to reduce transition noise, the size of the magnetic crystal grains needs to be reduced and the shape of the transition region of the recording bits needs to be made as straight as possible.
Thus, to enhance the performance of a perpendicular magnetic recording medium, both the alignment variance of the magnetic recording layer and the size of the magnetic crystal grains need to be reduced. It is also known that the underlayer plays an important role in controlling magnetic properties, alignment, and crystallinity of a magnetic recording layer. It is well known that when a magnetic recording layer epitaxially grows with respect to an underlayer, for example, the size of the magnetic crystal grains follows the size of the crystal grains in the underlayer. Therefore, it is essential to reduce the size of crystal grains in the underlayer to reduce the size of crystal grains in the magnetic recording layer.
A single layer of titanium or a titanium alloy such as TiCr was once proposed for an underlayer of a perpendicular magnetic recording medium. Although the use of a titanium alloy can reduce the size of the crystal grains in the magnetic recording layer, other problems arise, such as a large variance of alignment and the generation of an initial growth layer with disordered crystallinity at an initial growth stage of the magnetic recording layer. Thus, materials for the underlayer that can replace the titanium alloy are being contemplated.
For the underlayers in the above-mentioned state of the art, the proposals have been disclosed including using a double-layer underlayer consisting of a ruthenium layer and a layer of an iron alloy, chromium, or a cobalt alloy (see for example Japanese Unexamined Patent Application Publication No. 2002-100030), and using a ruthenium underlayer and a soft magnetic backing layer of a CoFe alloy (see for example Japanese Unexamined Patent Application Publication No. 2002-298323). The above proposals can improve magnetic properties and the electromagnetic conversion performance by reducing noise due to the soft magnetic layer. Another proposal included a lamination of a first underlayer of CoPd, CoAl, or Ni3Al and a second underlayer of Ru, Mo, or Pt, to improve the recording performance of a perpendicular magnetic recording medium (see for example Japanese Unexamined Patent Application Publication No. 2003-228815).
The above proposals can provide good crystallinity and crystal isolation, and favorable magnetic performance with a thick underlayer of at least 30 nm. Nonetheless, if the underlayer is made thinner, the magnetic performance tends to degrade noticeably. With the requirement for higher recording density, the underlayer needs to be as thin as possible. To reduce the transition noise and enhance the recording density of a perpendicular magnetic recording medium, it is required to ensure sharp recording magnetic field, to make the transition of a recording bit as straight as possible, and to form a recording bit with a minimal size. This needs a minimum distance between the soft magnetic backing layer and the magnetic head. Thus, it is important to reduce the thickness of a nonmagnetic layer disposed between the soft magnetic backing layer and the magnetic recording layer in addition to reducing the thickness of a protective layer and the flying height of the magnetic head.
Merely reducing the thickness of an underlayer degrades crystal alignment of the magnetic recording layer and deteriorates magnetic isolation between the magnetic crystal grains, worsening magnetic performances. There still remains a need for further improving the performance of a perpendicular magnetic recording medium. The present invention addresses this need.